This application claims the priority benefit of Taiwan application serial no. 91125098, filed Oct. 25, 2002.
1. Field of Invention
The present invention relates to an under-ball-metallurgy layer. More particularly, the present invention relates to an under-ball-metallurgy layer structure capable of increasing the bonding strength between the bonding pad on a chip and a solder bump.
2. Description of Related Art
Flip chip bonding technique is a method of joining a chip with a substrate or a printed circuit board (PCB). The chip has an active surface with a plurality of bonding pads arranged into an array. Each bonding pad on the chip has an under-ball-metallurgy layer (UBM) and a solder bump. Hence, the solder bumps may connect with a corresponding set of contact pads on the substrate or printed circuit board when the chip is flipped over. Note that the flip chip technique is able to produce a high-pin-count package with a smaller overall area and a shorter circuit length. Consequently, most semiconductor manufacturers have adopted the flip chip technique to fabricate chip packages, especially the high pin count packages.
FIG. 1 is a schematic cross-sectional view of a portion of a conventional flip chip. As shown in FIG. 1, the flip chip 100 includes a chip 110, an under-ball-metallurgy layer 120 and a plurality of solder bumps 130 (only one is shown). The chip 110 has an active surface 112 with a passivation layer 114 and a plurality of bonding pads 116 thereon. The passivation layer 114 exposes the bonding pads 116. Note that the active surface 112 of the chip 110 is the side of the chip 110 having most active devices. The under-ball-metallurgy layer 120 is formed between the bonding pad 116 and the solder bump 130 to serve as a junction interface.
The under-ball-metallurgy layer 120 further includes an adhesion layer 122, a barrier layer 124 and a wettable layer 126. The adhesion layer 122 is fabricated using a material such as aluminum or titanium for boosting the bonding strength between the bonding pad 116 and the barrier layer 124. The barrier layer 124 is fabricated using a material such as nickel-vanadium alloy for preventing the diffusion of metallic atoms from a metallic layer above the barrier layer 124 to a metallic layer below the barrier layer 124 and vice versa. The wettable layer 126 is fabricated using a material such as copper for boosting the wetting capacity of the under-ball-metallurgy layer 120 towards solder bump 130 material. Note that lead-tin alloy is normally used to fabricate the solder bump 130 due to its greater overall bonding strength. However, because of possible pollution of the environment, lead-free solder material is often adopted. In general, both lead-containing and lead-free material contains tin as a principle ingredient.
If copper is a major constituent of the wettable layer 126, tin within the solder bump 130 may react with copper inside the wettable layer 126 during a reflow process and lead to the formation of inter-metallic compound (IMC), in other words, Cu6Sn5. Gradually, an inter-metallic compound (IMC) layer (not shown) is formed between the wettable layer 126 and the solder bump 130. In addition, if nickel-vanadium alloy is a major constituent of the barrier layer 124, tin within the solder bump 130 may first react with copper inside the wettable layer 126 to form an inter-metallic compound (IMC) Cu6Sn5. Thereafter, tin within the solder bump 130 may continue to react with nickel within the barrier layer 124 to form another inter-metallic compound Ni3Sn4. Note that tin within the solder bump 130 and nickel within the barrier layer 124 react to produce inter-metallic compound Ni3Sn4, which is lumpy and discontinuous, after a long reflow process. In the presence of the lumpy inter-metallic compound, bonding strength of the solder bump 130 to the underlying under-ball-metallurgy layer 120 is weakened. Hence, the solder bump 130 may peel off from the chip leading to a deterioration of product reliability and yield.
Accordingly, one object of the present invention is to provide an under-ball-metallurgy layer between a bonding pad of a chip and a solder bump for reducing the growth rate of inter-metallic compound Ni3Sn4. Ultimately, bonding strength between solder bump and bonding pad can be maintained for a very long time and working life of associated chip package can be extended.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an under-ball-metallurgy layer structure between a bonding pad of a chip and a tin-based solder bump. The under-ball-metallurgy layer at least includes an adhesion layer over the bonding pad, a nickel-vanadium layer over the adhesion layer, a wettable layer over the nickel-vanadium layer and a barrier layer over the wettable layer. The barrier layer prevents the penetration of nickel atoms from the nickel-vanadium layer and reacts with tin within the solder bump to form inter-metallic compound. In addition, the barrier layer is fabricated using a material selected from a group consisting of nickel, iron and cobalt. Furthermore, the under-ball-metallurgy layer may include another wettable layer over the nickel-vanadium layer.
This invention also provides an alternative under-ball-metallurgy layer structure between a bonding pad of a chip and a tin-based solder bump. The under-ball-metallurgy layer at least includes an adhesion layer over the bonding pad, a wettable layer over the adhesion layer and a nickel-vanadium layer over the wettable layer. The nickel-vanadium layer may react with tin within the solder bump to form inter-metallic compound. Furthermore, the under-ball-metallurgy layer may include another wettable layer over the nickel-vanadium layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.